This invention relates generally to signal detection, and more particularly to calculating soft information for a signal obtained from a multi-level modulation system.
With the continuing demand for higher-speed digital communications systems and higher-density digital storage systems, various techniques have been applied to increase the capacity of these systems. However, even with high-capacity communications and storage media, their respective bandwidths and densities are still limited. Therefore, multi-level signaling can be applied to fully utilize the available bandwidth or density of these systems. For example, rather than binary signaling, e.g., 2-PAM, some communications system can increase the amount of information transmitted by using four signal levels, e.g., 4-PAM.
However, determining the information contained within a received signal of a multi-level system may be a complex problem. In particular, at each time interval, a received signal may correspond to a plurality of different digital information sequences, or “symbols.” Typically, to determine the actual symbol corresponding to a time interval of the received signal, soft information can be computed for each bit of the symbol. The soft information can correspond to the likelihood that a particular bit in the symbol is “0” or “1.” Soft information for the bits of a symbol are typically computed based on each possible value of the symbol. Because there can be many possible symbol values to consider in multi-level modulation schemes, computing soft information at each time interval can require a large amount of high-complexity computational circuitry. Furthermore, because these likelihoods may need to be computed frequently, the computational circuitry may also have high power consumption. Accordingly, it would be desirable to provide low complexity and low power techniques for calculating soft information in systems with multi-level modulation.